1. Technical Field
The present invention relates to a semiconductor device with a heatsink plate and to a method of manufacturing such semiconductor device, and more particularly to a technique of fixing the heatsink plate to the substrate.
2. Related Art
In recent years, a semiconductor device of a Ball Grid Array (hereinafter, BGA) package structure has been focused on because of the advantage it offers in achieving both higher integration level and superior performance.
To start with, a structure of the BGA semiconductor device will be reviewed. FIG. 5A is a vertical cross-sectional view schematically showing the structure of the BGA semiconductor device, and FIG. 5B is a schematic plan view thereof.
As shown in FIGS. 5A and 5B, the BGA semiconductor device includes a substrate 201 with a semiconductor chip 202 mounted thereon, such that the substrate 201 and the semiconductor chip 202 are electrically connected via a solder or a gold bump, with the gap therebetween encapsulated with an underfill resin 203. On the lower surface of the substrate 201 a plurality of electrodes electrically connected to the semiconductor chip 202 is formed, and on those electrodes solder bumps 204 are provided so as to serve as external electrodes. The solder bumps 204 are aligned in an array on the lower surface of the substrate 201, and hence a larger number of external electrodes can be obtained than in a Quad Flat Package (hereinafter, QFP) semiconductor device, which means that the package size can be reduced compared with the QFP semiconductor device, when the required number of external electrodes is the same.
The BGA semiconductor device is mounted at a predetermined position on a mounting substrate, and the BGA semiconductor device and the mounting substrate are subjected to heat treatment so as to reflow the solder bump 204, to thereby achieve electrical connection between the BGA semiconductor device and electrodes on the mounting substrate.
Meanwhile, the ongoing rapid diffusion of the internet and intranet has been creating demand for higher performance of the semiconductor chips mounted on the semiconductor devices. On the other hand, quicker operation of the semiconductor chip leads to increased heat generation from the circuit actions, thereby degrading reliability on the performance. Accordingly, techniques of providing a heatsink plate that dissipates the heat generated in the semiconductor chip have been proposed, so as to upgrade the reliability on the performance of the semiconductor device. To cite a few examples, a semiconductor device according to JP-A No. 2001-210761 includes a sheet-shaped heatsink plate constituted of a heat-resistant resin containing carbon fiber as a reinforcing material. JP-A No. 2001-168244 proposes employing a laser welding method for bonding a heat spreader (heatsink plate) to a semiconductor device. In a semiconductor device according to JP-A No. 2002-134669, a lid with radiation fins mounted thereon and an insulating substrate are adhered with adhesive resins of different Young's modulus. Also, JP-A No. 2004-165586 discloses a package structure, as well as a printed circuit board with the package mounted thereon, including a heat spreader that transmits heat from an LSI to a heat sink.
Accordingly, a structure of the BGA semiconductor device including the heatsink plate will be described. FIG. 6A is a vertical cross-sectional view and FIG. 6B is a plan view, each schematically showing a structure of a conventional BGA semiconductor device with a heatsink plate. FIG. 6C is an enlarged fragmentary vertical cross-sectional view of the BGA semiconductor device.
As shown in FIGS. 6A and 6B, the BGA semiconductor device includes a heatsink plate 205 overlaid on the semiconductor chip 202 mounted on the substrate 201, and the heatsink plate 205 is fixed to the upper surface of the semiconductor chip 202 via an adhesive resin or an adhesive agent 207 such as alumina paste or silver paste, and to the upper surface of the substrate 201 via an adhesive resin 206 as shown in FIG. 6C.
In such BGA semiconductor device, however, the package is warped as a whole during the reflow process of the solder bump 204. This is mainly because of a difference in thermal expansion coefficient between the semiconductor chip 202 and the substrate 201. In addition, a difference in thermal expansion coefficient between the semiconductor chip 202 and the underfill resin 203 may be partially responsible for the warp. More specifically, the greater thermal expansion coefficient of the substrate 201 and the underfill resin 203 than that of the semiconductor chip 202 is the cause of the warp.
Specific warp status of the BGA semiconductor device during the reflow process will be described hereunder. FIG. 7A is a vertical cross-sectional view schematically showing the conventional BGA semiconductor device warped during the reflow process (under high temperature), and FIG. 7B is an enlarged fragmentary vertical cross-sectional view thereof. FIG. 8A is a vertical cross-sectional view schematically showing the conventional BGA semiconductor device warped during a cooling phase of the reflow process (from low to room temperature), and FIG. 8B is an enlarged fragmentary vertical cross-sectional view thereof.
As shown in FIG. 7A, during the reflow process (under high temperature), the BGA semiconductor device suffers a warp in the direction as illustrated, because of the difference in thermal expansion coefficient between the semiconductor chip 202 and the substrate 201, and also the difference in thermal expansion coefficient between the semiconductor chip 202 and the underfill resin 203. At this moment, as shown in FIG. 7B, a stress is applied to the adhesive resin 206 joining the heatsink plate 205 and the substrate 201 from the lower surface of the heatsink plate 205 and the upper surface of the substrate 201, as indicated respectively by arrows A, B (in a compressing direction).
In contrast as shown in FIG. 8A, in the cooling phase of the reflow process (from low to room temperature), likewise, the BGA semiconductor device suffers a warp in the direction as illustrated, because of the difference in thermal expansion coefficient between the semiconductor chip 202 and the substrate 201, and also the difference in thermal expansion coefficient between the semiconductor chip 202 and the underfill resin 203. At this moment, as shown in FIG. 8B, a stress is applied to the adhesive resin 206 joining the heatsink plate 205 and the substrate 201 from the lower surface of the heatsink plate 205 and the upper surface of the substrate 201, as indicated by arrows A, B (in an elongating direction).
Thus, in the cooling phase of the reflow process (from low to room temperature), since the stress is applied to the adhesive resin 206 joining the heatsink plate 205 and the substrate 201 in the direction to strip off the heatsink plate 205 from the upper surface of the substrate 201 (elongating direction), the heatsink plate 205 is often stripped off from the upper surface of the substrate 201. When the heatsink plate 205 is thus stripped off from the upper surface of the substrate 201, the heatsink plate 205 becomes less closely pressed against the semiconductor chip 202, which leads to insufficient thermal contact between the heatsink plate 205 and the semiconductor chip 202. Besides, in case that the separation of the heatsink plate 205 from the upper surface of the substrate 201 further leads to emergence of a crack or fracture in the adhesive agent 207, which is now the only member retaining the heatsink plate 205, the thermal contact between the heatsink plate 205 and the semiconductor chip 202 becomes further deteriorated. Therefore, the heatsink plate 205 can no longer effectively dissipate the heat from the semiconductor chip 202, thus resulting a difficulty to secure the reliability on the performance of the BGA semiconductor device.
In the conventional BGA semiconductor device, as reviewed referring to FIGS. 7B and 8B, the lower surface of the heatsink plate 205 is fixed to the upper surface of the substrate 201 via the adhesive resin 206. Thus, while the anchor effect provided by the adhesive resin 206 is the only fixing means of the heatsink plate 205 to the substrate 201, it is not desirable to increase the footprint of the heatsink plate 205, from the viewpoint of reducing the package size as much as possible. Consequently, the structure of the conventional BGA semiconductor device result in a difficulty to maintain sufficient fixing strength of the heatsink plate 205 against the stress in the direction to strip off the heatsink plate 205 from the substrate 201 (elongating direction).